1. Field of the Invention
The present invention relates to a solid electrolyte and an all-solid state lithium ion secondary battery having Li ion conductivity.
2. Description of the Related Art
A Lithium ion secondary battery is an electrochemical device that stores and releases lithium by storing/detaching lithium with electron transfer in two electrode layers containing an active material capable of storing/detaching lithium ions.
Since a lithium ion that is a charge carrier has a small atomic number and high ionization tendency, lithium ion secondary batteries have higher energy density per volume and higher energy density per weight than other secondary batteries. Therefore, the lithium ion secondary batteries are widely used as a power source for portable devices, such as mobile phones or notebook PCs.
Further, applications to a power source for hybrid automobiles and electric automobiles, and a power source for power storage of power generation systems using renewable energy, such as photovoltaic power generation or wind power generation have been proceeding.
Here, many of the lithium ion secondary batteries currently put to practical use a flammable organic-solvent-electrolyte solution as an electrolyte. Therefore, there are risks of liquid leakage and ignition, and development of a highly safe lithium ion secondary battery without such risks is desired. As a battery without the risks of liquid leakage and ignition, all-solid state lithium ion secondary batteries that use non-flammable solid electrolytes having Li ion conductivity as an electrolyte have been developed in various places.
Among the all-solid state lithium ion secondary batteries, secondary batteries using a ceramic material having a structure that conducts lithium ions have excellent durability at a high temperature, and have drawn attention.
The ceramic material that conducts lithium ions is made of Li ions that serve as carriers and a polyanion framework having a space that serves as a pathway of the Li ions, and the ceramic materials are classified into various types according to constituent elements and a structure of the polyanion framework.
Currently, examples of the ceramic electrolyte material currently widely examined include a sulfide solid electrolyte that contains lithium-sulfur, phosphorus-sulfur, and transition metal-sulfur bonds in a polyanion, and an oxide solid electrolyte that contains lithium-oxygen, phosphorus-oxygen, and transition metal-oxygen bonds.
The sulfide solid electrolyte has a large atomic radius of sulfur and large polarization, and is thus suitable for conduction of lithium transfer. Further, the sulfide solid electrolyte is easily deformed by external pressure, and can increase contact areas among electrolyte particles or between the active material and electrolyte particles by compression at the time of manufacturing a battery. Therefore, a large number of all-solid state lithium secondary batteries have been examined using the sulfide solid electrolyte.
However, the sulfide solid electrolyte is extremely unstable in the atmosphere, and is decomposed by absorption of water and generates hydrogen sulfide that is a toxic gas. Therefore, there is room for improvement in terms of safety in manufacturing and in use.
Meanwhile, the oxide electrolyte is stable in the atmosphere and has excellent thermal durability. Therefore, the oxide electrolyte is promising as a highly safe electrolyte for all-solid state battery. Issues of the oxide electrolyte are high ion conductivity, wide chemical window, and especially, excellent reduction resistance.
As the oxide electrolyte having Li ion conductivity, a NASICON type glass ceramics Li1.5Al0.5Ge1.5(PO4)3 (LAGP) and Li1.3Al0.3Ti1.7(PO4)3 have been examined. However, it has been reported that reduction action is caused at from 0.5 to 2.4 V, both inclusive, with respect to a lithium electrode in each oxide electrolyte, and the ion conduction is impaired. Therefore, it cannot be said that the reduction resistance is high.
In contrast, a garnet type oxide made of lithium, lanthanum, zirconium, and the like has excellent reduction resistance because it is stable even if being in contact with lithium, and is a strong candidate of the solid electrolyte. In recent years, a garnet type oxide Li7La3Zr2O12 has been developed by a group of Weppner, et al., and it has been reported that the garnet type oxide has ion conductivity of 2.3×10−4 Scm−1, and activation energy of the ion conduction of 33 kJ/mol at room temperature.
Various element substitutions for Li, La, and Zr sites have been disclosed for improvement of the ion conductivity of the garnet type oxide Li7La3Zr2O12.
In JP 07-320971 A reports Li5+xLa3(Zrx,A2−x)O12 in which the zirconia site is substituted by an aliovalent cation, such as Ta or Nb, and reports that the activation energy of 30 kJ/mol and the ion conductivity of 8×10−4 S/cm can be obtained with the Nb substitution.
Further, JP 2012-224520 A discloses a composition in which the element Sr or Ca is substituted for the La site and the element Nb is substituted for the Zr site as compositions that obtain relatively high ion conductivity, even in a low sintering temperature, and reports that a sintered body at 1100° C. has the activation energy of 39 kJ/mol (0.40 eV) and the ion conductivity of 2.4×10−4S/cm.
Meanwhile, JP 2013-032259 A discloses a garnet type solid electrolyte in which the Li site is substituted by Al and Ga, and reports that the garnet type structure becomes stabilized by the element substitution of Al, Ga, and the like.